1. Field of the Invention
The present invention relates to modifying structures by a laser, and, more particularly, to removing material from photomasks and/or integrated circuits by the use of a pulsed laser.
2. Brief Description of Related Developments
During the manufacture of photomasks and/or integrated circuits, undesirable structures or parts of structures require removal or modification. Several techniques have been used to accomplish these objects in the past.
Almost all photomasks manufactured and especially, leading edge photomasks require the correction of defects that normally form as a result of excess chromium (Cr), for example, on the underlying substrate such as quartz (SiO2). The removal of such defects must not damage adjacent structures with the material removed by a laser, for example. This damage may be the result of splatter or haze created during the ablation of the material.
Several techniques have been used in the past. For example, a focused ion beam of gallium (Ga) with a halogen gas has been used. Spatial resolution of less than 25 nm has been achieved. Several disadvantages are Ga implanting as the undesired material is removed which significantly reduces the optical transmission of the underlying quartz surface. Also, the underlying quartz is almost always damaged by erosion and pitting. Another technique is the use of nanosecond pulsed laser beams to ablate the Cr, for example. The pulsed laser excites electrons whose energy is converted into phonons that subsequently heat the material. This heat may melt Cr that evaporates in a completely thermal process. As a result of thermal diffusion, the material may have balling or curling at the edges and splatters the material across the photomask surface near the ablation. The evaporated material produces a general haze, which reduces significantly the optical transmission of the quartz substrate. Also, the underlying quartz substrate is ablated and this creates an optical phase shift. Thus, thermal ablation from nanosecond pulses is not acceptable for repairing features having a size below 1 micron.
In order to avoid these problems in the past, the use of ultrashort (femtosecond) laser pulses have been used. This puts sufficient energy into the excited electrons to cause the material to turn into a plasma without the use of the thermal process. This non-thermal process does not degrade the resolution, no metal is splattered, no balling at the edges of the material, no damage to the substrate. A description of the femtosecond laser process and the photomask repair system is described in the article “MARS: Femtosecond laser photomask advanced repair system in manufacturing,” by Richard Haight, et al., published in Journal of Vacuum Science Technology, B 17(6), November/December 1999, pp. 3137 to 3143.